Light emitting diode (LED) chips emit a wide angular beam of light either from only a single top surface or from a plurality of surfaces. This results in a full beam angle which is at least 180 degrees wide. Since LEDs commonly emit a narrow spectrum of light it is necessary to use phosphor materials to convert the light emitted from the LED chip into a broader spectrum and higher wavelength (white light for instance).
For a number of lighting applications there is a requirement for a narrow beam of light. In some of these instances, space is also at a premium, for instance flash camera phones. In order to achieve a narrow beam of light in such instances, expensive low profile secondary optics are needed.
Conventional light emitting diodes allow heat dissipation through the surface area that is opposite to the top emitting surface area of the chip. The quantum wells layer where the heat is generated is separated from the heat dissipation area by a relatively thick carrier substrate (70 μm to 350 μm). As a result the thermal resistance of the LED chip is undesirably high.
An example of a method of manufacturing an LED chip (100) is provided in FIG. 1. In this example, layers of GaN are grown on a first substrate (105) which is made from silicon. The GaN layers consist of an n-type doped GaN layer (110), a layer comprising plurality of quantum wells (115) and p-type doped GaN layer (120) and is fabricated to include an n-GaN electrical contact (125), sidewall passivation (130) and a reflective p-GaN electrical contact (135) (see FIG. 1A). A second substrate (140), made from a silicon wafer, is bonded to the fabricated GaN structure using a permanent, conductive adhesive (145) (see FIG. 1B).
The original growth substrate is removed, allowing the fabricated GaN structure to be flipped and processed such that the surface is textured (150) (See FIG. 10). The completed LED structure is then tested, singulated and placed into a package where connections to the anode are made by way of a wire bond and to the cathode by way of a conductive adhesive.
Most common method to dispense a phosphor material over an LED chip is to assemble the LED chip into a carrier/submount that contains a dam around the LED chip so the phosphor material can be well controlled and evenly dispensed. However, the assembly of the LED chip into a carrier comes at a relatively high cost.
Examples of collimating light are provided in, for example, U.S. Pat. No. 8,076,831. The method of U.S. Pat. No. 8,076,831 involves the shaping of monochromatic light. When phosphor is applied, light is scattered by the particles in the phosphor solution, thereby undoing any pre-collimation. In order to focus the light to a spot or shape the light, secondary optics are employed which are expensive, typically costing many times more than the LED. Secondary optics can then be attached to the surface of the package or placed around the package to collimate the light for the purpose of directing a light beam, in accordance with the requirements of the final application. Coupling light into secondary optics is an inefficient process as it comes with unavoidable optical losses.
Another solution (as described in WO2009022316) consists of mounting the LED chip on a submount with an optical element in the form of a reflector. Alternatively, the reflector may be part of a molded lead frame. The molded lead frame may be conventionally produced, e.g., from a patterned conductor materials, such as copper. Plastic is injection molded around the conductor material to form the molded lead frame, and is also molded to form the optical element, e.g., reflector.
Low profile secondary optics are used to collimate the emitted light into a narrow beam but because of their relatively small pupil aperture, they suffer from lower performance, particularly when the size of the pupil aperture is only few times the size of the LED chip. In addition, high alignment tolerances are necessary between the LED chip and the optical axis of the secondary optical component.
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US2014/231842 relates to a semiconductor light emitting device and light emitting device.
Accordingly, it is desirable to provide an improved method of forming an LED chip and/or tackle at least some of the problems associated with the prior art or, at least, to provide a commercially useful alternative thereto.